Electron Coherence Length Research Articles

Nonvolatile ferroelectric control of electronic properties of Bi2Te3

Abstract Nonvolatile electric-field control of the unique physical characteristics of topological insulators (TIs) is essential for fundamental research and development of practical electronic devices. Electrically tunable transport properties through electrostatic gates have been extensively investigated. However, the relatively weak and volatile tunability limits its practical applications in spintronics. Here, we demonstrate the nonvolatile electric-field control of Bi2Te3 transport properties via constructing ferroelectric Rashba architectures, i.e., 2D Bi2Te3/α-In2Se3 ferroelectric field effect transistors. Through switching the polarization states of the α-In2Se3, the Fermi level, resistance, Fermi wave vector, carrier mobility, carrier density, and magnetoresistance (MR) of the Bi2Te3 film can be effectively modulated. Importantly, a shift of Fermi level toward band gap with surface state occurs as switching to negative polarization state, the contribution of the surface state to the conductivity increases, thereby increasing carrier mobility and electron coherence length significantly, and resulting in enhanced WAL effect. These results provide a nonvolatile electric-field control method to tune the electronic properties of TI and can further extend to the quantum transport properties.

Reply to “Comment on ‘Nonlinear quantum effects in electromagnetic radiation of a vortex electron' ”

We argue that although the experiment of Remez et al. [Phys. Rev. Lett. 123, 060401 (2019)] is interesting, and its conclusions may well be correct, the observed lack of dependence of the measured angular distributions on the electron's transverse coherence length could have been expected for the parameters chosen. This is because for Smith-Purcell radiation it is the coherence length of a virtual photon ${\ensuremath{\sigma}}_{\ensuremath{\perp}}^{(\ensuremath{\gamma})}\ensuremath{\approx}\ensuremath{\beta}\ensuremath{\gamma}\ensuremath{\lambda}\ensuremath{\lesssim}\ensuremath{\lambda}$ that plays a role of the radiation formation width and not the entire electron's coherence length that can well be orders of magnitude larger than the former. This is a common feature for all the radiation processes in which a photon is emitted not directly by the electron packet, which can be delocalized in space, but rather by a much better localized atom or a conduction electron on a surface. Therefore, in our opinion the results of Remez et al. cannot rule out the alternative hypothesis of the delocalized charge. The question, mainly addressed in the Comment by Karnieli et al., of whether the measurements were performed in the wave zone or not is interesting but somewhat secondary. We emphasize that the measured azimuthal distributions are unusually wide and there exists a family of classical effects that could also have resulted in the measured distributions. Such alternative classical hypotheses include: (i) effects of the beam sizes, of its angular divergence, of the temporal coherence of the radiation process, which is also related to how the wave zone is defined, and (ii) influence of the grating shape and of its material---the effects that are known to be of crucial importance for Smith-Purcell radiation from nonrelativistic electrons. Finally, we propose to repeat the experiment and to measure diffraction radiation from a thin metallic semiplane (or a strip) in which case the aforementioned classical effects play a much smaller role.

Flicker noise in two-dimensional electron gas

Using the method developed in a recent paper (2019 Euro. Phys. J. B 92 1–28) we consider 1/f noise in two-dimensional electron gas (2DEG). The electron coherence length of the system is considered as a basic parameter for discretizing the space, inside which the dynamics of electrons is described by quantum mechanics, while for length scales much larger than it the dynamics is semi-classical. For our model, which is based on the Thomas-Fermi–Dirac approximation, there are two control parameters: temperature T and the disorder strength (Δ). Our Monte Carlo studies show that the system exhibits 1/f noise related to the electronic avalanche size, which can serve as a model for describing the experimentally observed flicker noise in 2DEG. The power spectrum of our model scales with the frequency with an exponent in the interval 0.3 < α PS < 0.6. We numerically show that the electronic avalanches are scale-invariant with power-law behaviors in and out of the metal-insulator transition line.

Coulomb interactions and effective quantum inertia of charge carriers in a macroscopic conductor

We study the low frequency admittance of a quantum Hall bar of size much larger than the electronic coherence length. We find that this macroscopic conductor behaves as an ideal quantum conductor with vanishing longitudinal resistance and purely inductive behavior up to f<1MHz. Using several measurement configurations, we study the dependence of this inductance on the length of the edge channel and on the integer quantum Hall filling fraction. The experimental data are well described by a scattering model for edge magnetoplasmons taking into account effective long range Coulomb interactions within the sample. This demonstrates that the inductance's dependence on the filling fraction arises from the effective quantum inertia of charge carriers induced by Coulomb interactions within an ungated macroscopic quantum Hall bar.

Synthesis and applications of silver nanocomposites: A review

Nanomaterials refer to materials whose structural unit size is between 1 nanometer and 100 nanometers. Since nanoparticles’ size is close to the coherence length of electrons, their properties have also changed greatly due to the self-organization brought about by strong coherence. Therefore, noble metal nanoparticles have unique physical, chemical, and biological properties. This paper mainly studies the silver nanoparticle material, one of the precious metal nanoparticles. The silver nanoparticle is modified to graphene, metal materials, fiber materials, ceramic materials, and polymers to form a silver-based composite material, which improves its antibacterial, electrical conductivity, and Chemical durability, photocatalysis, and other capabilities. They can be applied to medical, environmental, industrial, biological, food and other fields, providing a reference for in-depth research on the properties of nano-silver particles and the continuous development of their application prospects.

Unraveling the effect of electron-electron interaction on electronic transport in La-doped SrSnO3 films

We demonstrate that the electron-electron interaction effect is primarily responsible for an increase in the Hall coefficient in the La-doped SrSnO3 films below 50 K accompanied by an increase in the sheet resistance. The quantitative analysis of the magnetoresistance data yielded a large phase coherence length of electrons exceeding 450 nm at 1.8 K and revealed the electron-electron interaction being accountable for the breaking of electron phase coherency in La-doped SrSnO3 films. These results while providing critical insights into the fundamental transport behavior in doped stannates also indicate the potential applications of stannates in quantum coherent electronic devices owing to their large phase coherence length.

Macroscopic Electron Quantum Coherence in a Solid-State Circuit

The quantum coherence of electronic quasiparticles underpins many of the emerging transport properties of conductors at small scales. Novel electronic implementations of quantum optics devices are now available with perspectives such as 'flying' qubit manipulations. However, electronic quantum interferences in conductors remained up to now limited to propagation paths shorter than $30\,\mu$m, independently of the material. Here we demonstrate strong electronic quantum interferences after a propagation along two $0.1\,$mm long pathways in a circuit. Interferences of visibility as high as $80\%$ and $40\%$ are observed on electronic analogues of the Mach-Zehnder interferometer of, respectively, $24\,\mu$m and $0.1\,$mm arm length, consistently corresponding to a $0.25\,$mm electronic phase coherence length. While such devices perform best in the integer quantum Hall regime at filling factor 2, the electronic interferences are restricted by the Coulomb interaction between copropagating edge channels. We overcome this limitation by closing the inner channel in micron-scale loops of frozen internal degrees of freedom, combined with a loop-closing strategy providing an essential isolation from the environment.

Electronic structure of self-assembled Ag nanowires on Si(557): spectroscopic evidence for dimensionality

We investigate the electronic structure of silver nanowires at 1 monolayer (ML) Ag coverage on a vicinal Si(557) substrate by angle-resolved photoemission spectroscopy. Fermi surface mapping reveals two-dimensional (2D) Ag-induced electron pockets, which shift to higher binding energies by n-type doping of the step edges. The electronic structure of the 1 ML Ag/Si(557) system exhibits additional 2D metallic states near the Fermi level, which may originate from substrate induced periodicities. By considering the confining potential and electronic coherence length we reconcile the seemingly conflicting views of the dimensionality in this system probed by photoemission spectroscopy, and the dispersion of collective excitations as detected by electron energy loss spectroscopy .

Structural analysis of polycrystalline graphene systems by Raman spectroscopy

A theoretical model supported by experimental results explains the dependence of the Raman scattering signal on the evolution of structural parameters along the amorphization trajectory of polycrystalline graphene systems. Four parameters rule the scattering efficiencies, two structural and two related to the scattering dynamics. With the crystallite sizes previously defined from X-ray diffraction and microscopy experiments, the three other parameters (the average grain boundaries width, the phonon coherence length, and the electron coherence length) are extracted from the Raman data with the geometrical model proposed here. The broadly used intensity ratio between the C–C stretching (G band) and the defect-induced (D band) modes should be used to measure samples with crystallite sizes larger than the phonon coherence length, which is found equal to 32 nm. The Raman linewidth of the G band is more appropriate to characterize the crystallite sizes below the phonon coherence length, down to the average grain boundaries width, which is found to be 2.8 nm. “Ready-to-use” equations to determine the crystallite dimensions based on the Raman spectroscopy data are given.

Magnetoresistance oscillations in topological insulator Bi2Te3 nanoscale antidot arrays

Nanoscale antidot arrays were fabricated on a single-crystal microflake of topological insulator Bi2Te3. The introduction of antidot arrays significantly increased the resistance of the microflake, yet the temperature dependence of the resistance remains metallic. We observed that small oscillations that are periodic in magnetic field B appeared on top of the weak anti-localization magnetoresistance. Since the electron coherence length at low temperature becomes comparable to the feature size in our device, we argued that the magnetoresistance oscillations are the manifestation of quantum interference induced by the nanostructure. Our work demonstrates that the transport of topological insulators could indeed be controlled by artificially created nanostructures, and paves the way for future technological applications of this class of materials.

Probing the conductance superposition law in single-molecule circuits with parallel paths

According to Kirchhoff's circuit laws, the net conductance of two parallel components in an electronic circuit is the sum of the individual conductances. However, when the circuit dimensions are comparable to the electronic phase coherence length, quantum interference effects play a critical role, as exemplified by the Aharonov-Bohm effect in metal rings. At the molecular scale, interference effects dramatically reduce the electron transfer rate through a meta-connected benzene ring when compared with a para-connected benzene ring. For longer conjugated and cross-conjugated molecules, destructive interference effects have been observed in the tunnelling conductance through molecular junctions. Here, we investigate the conductance superposition law for parallel components in single-molecule circuits, particularly the role of interference. We synthesize a series of molecular systems that contain either one backbone or two backbones in parallel, bonded together cofacially by a common linker on each end. Single-molecule conductance measurements and transport calculations based on density functional theory show that the conductance of a double-backbone molecular junction can be more than twice that of a single-backbone junction, providing clear evidence for constructive interference.

Surface morphology and transport studies of epitaxial graphene on SiC(0001̲)

Annealing conditions are critical to the properties of epitaxial graphene formed by thermal decomposition of silicon carbide. Here, we report the evolution of coherent electronic transport with increasing anneal temperatures, combined with low energy electron micrographs of equivalent surfaces showing corresponding structural coherence. Ultrahigh vacuum conditions and temperatures in the range of 1250--1300 ${}^{\ifmmode^\circ\else\textdegree\fi{}}$C produce granular films with a lateral grain size of $~$20 nm, while temperatures of 1400 ${}^{\ifmmode^\circ\else\textdegree\fi{}}$C or higher result in grains with progressively larger lateral dimensions in the micron range. Transport measurements show how the electronic coherence length increases as a result of the more coherent physical structure, with a crossover from two-dimensional variable range hopping to the weak localization regime. Here, we show that while the duration of the anneal affects coverage and clustering of grains, the size of individual grains is determined by anneal temperature, with evidence of coalescence of smaller grains into larger domains, suggesting that multistage anneals at different temperatures may yield high-quality graphene.

Low Temperature Raman Study of the Electron Coherence Length near Graphene Edges

We developed a novel optical defocusing method for studying spatial coherence of photoexcited electrons and holes near edges of graphene. Our method is applied to measure the localization l(D) of the disorder-induced Raman D band (∼1350 cm(-1)) with a resolution of a few nanometers. Raman scattering experiments performed in a helium bath cryostat reveal that as temperature is decreased from 300 to 1.55 K, the length l(D) increases. We found that the localization of the D band varies as 1/T(1/2), giving strong evidence that l(D) scales with the coherence length of photoexcited electrons near graphene edges.

Quantum interference Hall effect in nanopatterned two-dimensional electron gas systems

We show that in mesoscopic two-dimensional electron gas systems, quantum interference caused by prede- signed nanopatterns can enhance the Hall effect by up to 500%. A quality factor Q is defined which optimizes the ratio of the Hall voltage to longitudinal voltage. Genetic algorithm and the Landauer-Buttiker formalism were used to search for the potential configuration that can achieve a large Q. We propose some realistic nanopatterns to realize this effect. Quantum Hall effect shows that electron's quantum wave nature can alter its classical Hall character under a strong magnetic field. 1 But can such wave characteristics persist in the limit of B →0? Some time ago it was shown that the giant Hall effect GHE in magnetic materials can persist even in the limit where magnetization has reached saturation and therefore cannot play a role. 2 It follows that there could be some alternative scenario for the enhancement of the Hall effect that are distinct from the usual GHE. 2,3 Motivated by this finding, subsequent experimental and theoretical studies on the nonmagnetic CuSiO2 composites have found that quantum interference may indeed play a crucial role in en- hancing the Hall coefficient. 4,5 This conclusion was sup- ported by the fact that only when the size of the metallic granules is much smaller than the coherent length of the electron, e.g., in the low-temperature regime so that the electron coherence length can reach its saturation value, can this GHE occur. Most dramatically, the effect vanishes in annealed samples, when the size of the metallic granules exceeds the quantum coherence length. Together with the fact that all the GHE was observed in the vicinity of the percolation threshold, it was concluded that the GHE arises from the quantum interference effect in the percolation geo- metric setting. By using mesoscopic transport calculations in the microscopic regions within a coherence length to evalu- ate the local Hall conductances, Wan and Sheng 5 built a model in which the local mesoscopic Hall conductance ten- sors thus obtained served as the inputs to the macroscopic classical electric network calculations. The results show ex- cellent agreement with the experiment data. The random nature of the composite geometry means that the GHE is manifest only in a statistically averaged sense. In this paper, we show in mesoscopic two-dimensional 2D electron gas systems one can utilize predesigned nanostruc- tures to maximize the Hall effect in the limit of small mag- netic field. Genetic algorithm was used to search the optimal 2D structure pattern. It is shown that for certain nanopat- terned configurations the Hall coefficient can be enhanced up to 300%-500% in 2D electron systems with parameters rel- evant to semiconductor heterostructures. By using designed nanostructures, the case for quantum interference Hall effect can be much better verified, as there can be a direct correla- tion between the nanostructure and the measured Hall char- acteristics. In what follows, the Hall effect in the mesoscopic context, i.e., in nanostructures, is described in Sec. II together with its calculational approach. In Sec. III we present the results on the Hall effect in homogeneous mesoscopic samples in order to set the stage for the consideration of patterned samples. In Sec. IV the definition of a quality factor for measuring the mesoscopic Hall effect is followed by pattern optimizations to achieve maximum Hall-effect enhancement. A few opti- mal nanopatterns are presented and their relevant parameter values compared with those of the homogeneous samples.

One-Dimensional Weak Localization of Electrons in a Single InAs Nanowire

We report on low temperature (2-30K) electron transport and magneto-transport measurements of a chemically synthesized InAs nanowire. Both the temperature, T, and transverse magnetic field dependences of the nanowire conductance are consistent with the functional forms predicted in one-dimensional (1D) weak localization theory. By fitting the magneto-conductance data to theory, the phase coherence length of electrons is determined to be tens of nanometers with a T(-1/3) dependence. Moreover, as the electron density is increased by a gate voltage, the magneto-conductance shows a possible signature of suppression of weak localization in multiple 1D subbands.

Electron coherence length and mobility in highly mismatched III-N-V alloys

We investigate the quantum coherence length, Lφ, and mobility of conduction electrons in the dilute nitride alloy GaAs1−xNx. Analysis of the negative magnetoresistance using weak localization theory reveals a marked reduction in Lφ with increasing N-content. Our data are compared to theoretical models of electronic transport in GaAs1−xNx and discussed in terms of the unusual type of compositional disorder exhibited by III-N-V alloys. A comparative study of the electron mobility in InAs1−xNx also indicates that disorder effects are significantly weaker in this small band gap material.

Effect of phase‐breaking events on electron transport in mesoscopic and nanodevices

AbstractExisting ballistic models for electron transport in mesoscopic and nanoscale systems break down as the size of the device becomes longer than the phase coherence length of electrons in the system. Krstic et al. experimentally observed that the current in single‐wall carbon nanotube systems can be regarded as a combination of a coherent part and a noncoherent part. In this article, we discuss the use of Büttiker phase‐breaking technique to address partially coherent electron transport, generalize that to a multichannel problem, and then study the effect of phase‐breaking events on the electron transport in two‐terminal graphene nanoribbon devices. We also investigate the difference between the pure‐phase randomization and phase/momentum randomization boundary conditions. While momentum randomization adds an extra resistance caused by backward scattering, pure‐phase randomization smooths the conductance oscillations because of interference. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

Switching behaviour and high frequency response of amorphous carbon double-barrier structures

In order to produce fast memory switching devices fabrication of double-barrier resonant tunnel diodes (DB-RTD) based on amorphous semiconductors with a prominent signature of resonant tunnelling and negative differential resistance (NDR) was attempted. We have developed DB-RTDs of amorphous carbon and showed unique features including NDR peaks, quantized conductance and switching effects in these structures. From the analysis of these experimental data the effective mass of electron of about 0.07 times the free electron mass, a high value of mobility and long coherence length of electrons in this low dimensional disordered system is derived. At higher operational bias voltage we recorded periodic current steps, which are explained by the creation of 1D channels in this structure. These features suggest ballistic conduction. We are able to tune the switching characteristics of these structures in the presence of microwave frequency at about 100 GHz which resembles a quantum well capacitor with a fast relaxation time. The increase of conductance at a particular bias, in the presence of microwave frequency can be explained through an increase of mobility and increased relaxation time of electrons. A carbon-based memory suitable for very fast operation, compared to other organic polymer systems is suggested.

Phenomenology of conduction in incoherent layered crystals

A novel phenomenological approach to the analysis of the conductivities of incoherent layered crystals is presented. It is based on the fundamental relationship between the resistive anisotropy $\sigma_{ab}/\sigma_c$ and the ratio of the phase coherence lengths in the respective directions. We explore the model-independent consequences of a general assumption that the out-of-plane phase coherence length of single electrons is a short fixed distance of the order of interlayer spacing. Several topics are discussed: application of the scaling theory, magnetoresistivity, the effects of substitutions and the intermediate regime of conduction when both coherence lengths change with temperature, but at different rate.

DETECTION OF SPIN INJECTION EFFICIENCY BY TUNNELING MAGNETORESISTANCE

To detect the spin injection efficiency η from a ferromagnet metal (FM) to a semiconductor (SC) in an FM/SC tunnel junction, we propose to introduce another similar single junction to form an FM/SC/FM double tunnel junction, and to measure its tunneling magnetoresistance ( TMR ). It is found that there is an approximate relation TMR = η2 in the sequential tunneling region and at low bias voltage. As the thickness of the middle SC layer is decreased to the extent so that the size of the entire structure is smaller than the electronic coherence length, both coherent tunneling conductance and resonant TMR exhibit oscillating behavior.