Phase Coherence Length Lφ Research Articles

Charge transport in n-type As- and Sb-hyperdoped Ge

This paper presents a systematic study of the charge transport behavior of heavily doped n-type Ge layers with As and Sb. A nonequilibrium method ion implantation followed by milliseconds flash lamp annealing is applied to synthesize the n++ Ge layers (Ge:As and Ge:Sb). The resulting materials contain free electrons with a density above 3 × 1019 cm−3 and mobility more than 220 cm2/(V s). Quantum corrections to the conductance in a magnetic field are observed at low temperatures. Weak localization persists up to 30 K in Ge:Sb, while only up to 10 K in Ge:As. Using the Hikami–Larkin–Nagaoka model to fit the magnetoconductance data, we obtain the phase coherence length lϕ of the hyperdoped Ge samples in the range of 70–163 nm. This study may pave a way to explore possible applications for quantum technologies utilizing As- and Sb-hyperdoped Ge.

Magnetotransport and thermoelectric properties of cobalt doped Bi2Te3 nanostructures

Magnetotransport and thermoelectric properties of Bi2-xCoxTe3 nanomaterials have been studied. Temperature-dependent electrical resistivity of the materials shows a drastic change from degenerate semiconductor to a metal-like behavior with incorporation of cobalt in the sample. The observation of T and ln(T) dependent electrical conductivity is predicted to have electron–electron interaction (EEI), and quantum interference effect (QIE) dominated transport mechanisms respectively. Low field MR of the materials exhibits sharp dips demonstrating the appearance of QIE, and a crossover from linear MR at low temperature to linear plus quadratic MR at high temperature indicates the quantum transport of the materials. Quantum correction of the low field magnetoconductivity follows 2D Hikami-Larkin-Nagaoka equation and the temperature variation of phase coherence length lφ indicates the existence of 2D transport in the materials. The decrease of phase coherence length and increase of α parameter with the incorporation of cobalt leads to the enhancement of bulk domination of the conduction.

Comprehensive analysis for the high field magneto-conductivity of Bi2Te3 single crystal

Here, we report the magneto-conductivity (up to 14 T and down to 5 K) analysis of Bi2Te3 single-crystal. A sharp magneto-conductivity (MC) rise (inverted v-type cusp) is observed near H = 0 due to the weak antilocalization (WAL) effect, while a linear curve is observed at higher fields. We account for magneto-conductivity (MC) over the entire range of applied magnetic fields of up to 14 T and temperatures from 100 K to 5 K in a modified HLN modelling (addition of quadratic (βH2) through quantum and classical components involvement. The additional term βH2 reveals a gradual change of a (HLN parameter) from −0.421(6) to −0.216(1) as the temperature increases from 5 K to 100 K. The phase coherence length Lϕ obtained from both conventional and modified modelling decreased with increasing temperature but remains more protracted than the mean free path (L) of electrons. The analysis shows that the quantum phase coherence effect dominates at high temperatures.

Quasi-2D Transport and Weak Antilocalization Effect in Few-layered VSe2.

With strong spin-orbit coupling (SOC), ultrathin two-dimensional (2D) transitional metal chalcogenides (TMDs) are predicted to exhibit weak antilocalization (WAL) effect at low temperatures. The observation of WAL effect in VSe2 is challenging due to the relative weak SOC and three-dimensional (3D) transport nature in thick VSe2. Here, we report on the observation of quasi-2D transport and WAL effect in sublimed-salt-assisted low-temperature chemical vapor deposition (CVD) grown few-layered high-quality VSe2 nanosheets. The WAL magnitudes in magnetoconductance can be perfectly fitted by the 2D Hikami-Larkin-Nagaoka (HLN) equation in the presence of strong SOC, by which the spin-orbit scattering length lSO and phase coherence length lϕ have been extracted. The phase coherence length lϕ shows a power law dependence with temperature, lϕ∼T-1/2, revealing an electron-electron interaction-dominated dephasing mechanism. Such sublimed-salt-assisted growth of high-quality few-layered VSe2 and the observation of WAL pave the way for future spintronic and valleytronic applications.

Spin-related phenomena in nanoscale Si < B, Ni> whiskers

Abstract Transverse magnetoresistance in p-type conductivity Si whiskers with different impurity concentration that correspond to the dielectric side of metal-insulator transition were studied in magnetic fields 0–8 T at low temperatures 1.6 to 50 K. The presence of negative magnetoresistance in Si whiskers with concentration 2 × 1018 cm−3 was observed and associated with weak localization. The obtained parameters of phase coherence length lφ and spin-orbit coherence length lso comprise approximately 45 nm and 750 nm, respectively, at 4.2 K. The parameters obtained for Si whisker indicate the presence of hopping conductivity, orders of magnitude stronger than the parameters obtained for Si whisker with variable-range conductance, which indicates substantial impact of Ni impurities on the whisker magnetoresistance.

Effect of porosity on the enhancement of relative magnetoconductance in porous Si nanostructures: a comparative study with Si nanowires

We reported a comparative study of relative magnetoconductance (RMC) effect between porous silicon nanostructures (PSiNs) and silicon nanowires (SiNWs). We investigated the correlation between the RMC and the porosity in PSiNs produced by an electrochemical etching process in HF-based solutions. The experimental results exhibit a positive RMC in PSiNs which depends heavily on the porosity. The RMC effect passes from 0.09 to 2.2% for porosities ranging from 32 to 55% at low static magnetic field of 0.5 T at room temperature. SiNWs samples were formed by metal-assisted chemical etching method of crystalline silicon in AgNO3-based chemical solutions. Scanning electron microscopy shows that the SiNWs may have different structures depending on the etching time. Comparatively to PSiNs, the observed positive RMC was enhanced in SiNWs, and this effect is important and reached 9% at a magnetic field of 0.5 T. Weak localization (WL) theory was used to explain the observed positive RMC and to determine the phase coherence length L ϕ. We found that WL effect can be observed at room temperature on inhomogeneous nanostructured thin films and the L ϕ length depends heavily on the porosity effect in PSiNs.

Quantum electrical transport properties of topological insulator Bi2Te3 nanowires

We investigate the quantum transport properties of surface electrons on a topological insulator Bi2Te3 nanowire in a magnetotransport study. Although the nanowires are synthesized by using a relatively coarse method of electrochemical deposition, clear Aharonov–Bohm oscillations of phases 0 and π are observed, owing to the highly coherent surface electron channel. The oscillation amplitude exhibits exponential temperature dependence, suggesting that the phase coherence length Lφ is inversely proportional to the temperature, as in quasi-ballistic systems. In addition, a weak antilocalization analysis on the surface channel by using a one-dimensional localization theory, enabled by successful extraction of the surface contribution from the magnetoconductance data, is provided in support of the temperature dependence of Lφ.

Electrical conductance and magnetoconductance effect on n-type porous silicon: Contribution of weak localization correction

We report the electrical conductance and magnetoconductance (MC) effect in n-type porous silicon (PS) layers. The samples show significant positive MC at room temperature and the presence of a memory effect when a perpendicular low static magnetic field is applied (first scan: 0T→0.8T, second scan: 0.8T→0T). We modeled the experimental results in terms of quasi-one-dimensional (quasi-1D) weak localization (WL) theory. From the fit to experimental data we determined some parameters such as the phase coherence length Lϕ, the mean free path le and the Fermi wave vector kF for different porosities. These results seem to indicate that WL effect at room temperature predominate on these inhomogeneous systems.

Weak localization effect in pulsed laser deposited ZnO film

Low temperature longitudinal magnetoresistance measurement in pulsed laser deposited undoped ZnO film has been investigated. The temperature and magnetic field dependence of the resistivity of the film is in good agreement with the theory of weak localization and electron-electron interactions in a disordered system. Negative magnetoresistance is a consequence of the suppression of quantum interference effect in the weak localization theory. The inelastic scattering time τi ~ T −3/2 and phase coherence length Lϕ(T) ~ T −3/4 indicates that the electron-electron interaction is the dominant inelastic scattering process.

Phase coherent transport in GaAs/AlGaAs core–shellnanowires

We report on low temperature magneto-conductance measurements of a single modulation-doped GaAs/AlGaAs core–shell nanowire in transverse magnetic field. Information on the phase coherence length lϕ is obtained from quantum conductivity corrections due to weak localization effects or universal conductance fluctuation. The nanowires are totally coherent at very low temperature and lϕ could be larger than 250nm at 4K. The temperature dependence of lϕ shows that the main dephasing mechanism is the one-dimensional electron–electron scattering mechanism.

Observation and study of the negative magnetoresistance in porous silicon

We investigate the effect of a static magnetic field on the electrical properties of thin films of porous silicon. We observe a relative variation of the resistance with respect to intensity of the applied magnetic field. This effect called magnetoresistance (MR) has been found to be negative at room temperature in the entire range of the applied magnetic field (under 8000G). The negative MR effect depends strongly of the applied voltage and reaches up to 5% at the value of magnetic field of 8000G. The analysis has been based on the 1D weak localization theory (WL) in diffusive semiconductors, from which a consistent picture emerges, considering the PS film as networks of quasi-one-dimensional wires. We give an explicit expression for the WL correction depending only of the phase coherence length Lφ. A good agreement is observed between experimental data and theory.

Porosity dependence of positive magnetoconductance in n‐type porous silicon

AbstractPositive magnetoconductance (MC) on n‐type porous silicon (PS) based devices was observed at room temperature for low static magnetic field (under 6000 G). We found that the measured MC decreases when the film porosity is increased. Obtained results were analyzed by means of the quasi‐1D weak localization (WL) theory. From the dependence of the MC vs. applied magnetic field, we determine the phase coherence length Lϕ. Good agreement between theoretical and experimental results was found (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Phase coherent transport in (Ga,Mn)As

Quantum interference effects and resulting quantum corrections of the conductivity have been intensively studied in disordered conductors over the last few decades. The knowledge of phase coherence lengths and underlying dephasing mechanisms are crucial in understanding quantum corrections to the resistivity in different material systems. Due to the internal magnetic field and the associated breaking of time-reversal symmetry quantum interference effects in ferromagnetic materials have scarcely been explored. Below, we describe the investigation of phase coherent transport phenomena in the newly discovered ferromagnetic semiconductor (Ga,Mn)As. We explore universal conductance fluctuations in mesoscopic (Ga,Mn)As wires and rings, the Aharonov–Bohm effect in nanoscale rings and weak localization in arrays of wires, made of the ferromagnetic semiconductor material. These experiments allow to probe the phase coherence length Lϕ and the spin flip length LSO as well as the temperature dependence of dephasing.

Electron phase coherence length in a lattice of antidots

The temperature dependence of the phase coherence length Lϕ of electrons in a two-dimensional hexagonal lattice of antidots is investigated through weak localization (WL) measurements over a wide temperature range from 40mK to 30K. The magnetic field dependence of the WL can be perfectly described by the conventional theory for disordered conductors, with an additional magnetic-field-independent prefactor α. Using a two parameters fit of the WL, Lϕ can be extracted. An unusual T−1/4 dependence was found for Lϕ, which is contradictory with most of the models proposed for phase breaking processes. A saturation of Lϕ was observed below 1K. Finally, α was found to scale exponentially with the temperature. Surprisingly, the values obtained for α were particularly high, revealing mechanisms which enhance the WL in our system.

Phase Coherence Length in Planar Doped Thin GaAs Wires Fabricated by Ion Beam Etching

Electrical transport properties have been studied in a film and wires fabricated from a monolayer doped or a homogeneously doped thin GaAs by mesa etching techniques. The dependence of the electron wave coherency upon the wire widths and fabrication processes is investigated from their low-temperature behavior of the phase coherence length Lφ derived from the nagative magnetoresistance and the amplitude of the conductance fluctuations. Low-temperature saturations of Lφ occur only in narrow wires. This suggests that Lφ is affected by the ion beam induced damage within the mesa side walls. A difference in Lφ between reactive ion-beam-etched and ion-milled wires is explained by the decrease of the diffusion constant. The corresponding difference in the conductance fluctuation amplitudes is larger than that expected from the difference in Lφ.

Saturation of phase coherence length in GaAs/AlGaAs on-facet quantum wires

Phase coherence length Lφ of on-facet quantum wires is determined at temperatures down to 50 mK by three methods: magnetoresistance, conductance fluctuation amplitude, and the conductance fluctuation correlation field. It is found that Lφ saturates below about 0.4 K. This saturation is identified as due to spin-orbit interaction, which has no temperature dependence. This result is supported by positive magnetoresistance under a weak magnetic field.

Phase coherence length of electron waves in narrow AlGaAs/GaAs quantum wires fabricated by focused ion beam implantation

The phase coherence length Lφ of electron waves in the one-dimensional weak localization regime was studied in selectively doped AlGaAs/GaAs quantum wires fabricated by focused ion beam implantation. Estimated Lφ by fitting the modified weak localization theory to the data is ∼1.2 μm at 0.3 K, nine times longer than in the n-GaAs wires. This difference is well explained by the mobility dependence of Lφ, and shows the advantage of selectively doped structures to obtain long Lφ. Lφ increased with decreasing temperature and saturated below ∼3 K, indicating the existence of temperature-independent phase breaking mechanisms.