Phase Coherence Length 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.

Signatures of Mesoscopic Transport in Single Non‐Intentionally Doped GaN‐Nanowire Field‐Effect Transistors

In this work, the fabrication and characterization of a fully functional field‐effect transistor (FET) are addressed based on a non‐intentionally doped GaN‐nanowire FET (NW–FET). Universal conductance fluctuations (UCFs) are observed at temperatures below 140 K. In contrast to other reports in literature, UCFs appear in the analyzed NW–FET only under the influence of an electrical field when applying a gate voltage, while no UCF signatures are observed when performing magnetic‐field‐dependent measurements. The reason is the considerable impact of the applied voltage on the narrow conductive channel of the non‐intentionally doped NW. The electrical field influences the Fermi level as well as the width of the depletion region, both changing the effective impurity distribution which determines the set of possible electron paths. The electric‐field‐induced variation of the set of electron paths correlates with a conductance variation, which leads to the occurrence of UCFs. Furthermore, the reliability of determining the phase coherence length from the NW–FET transfer characteristics is analyzed. It is shown that the value of is significantly affected by the choice of the gate voltage range due to the current dependence of the magnitude of the UCFs.

Localization effect in single crystal of RuAs2

We report the magnetotransport and thermal properties of RuAs2 single crystal. RuAs2 exhibits semiconductor behavior and localization effect. The crossover from normal state to diffusive transport in the weak localization (WL) state and then to variable range hopping (VRH) transport in the strong localization state has been observed. The transitions can be reflected in the measurement of resistivity and Seebeck coefficient. Negative magnetoresistance (NMR) emerges with the appearance of localization effect and is gradually suppressed in high magnetic field. The temperature dependent phase coherence length extracted from the fittings of NMR also indicates the transition from WL to VRH. The measurement of Hall effect reveals an anomaly of temperature dependent carrier concentration caused by localization effect. Our findings show that RuAs2 is a suitable platform to study the localized state.

Universal Conductance Fluctuations in a MnBi2Te4 Thin Film.

Quantum coherence of electrons can produce striking behaviors in mesoscopic conductors. Although magnetic order can also strongly affect transport, the combination of coherence and magnetic order has been largely unexplored. Here, we examine quantum coherence-driven universal conductance fluctuations in the antiferromagnetic, canted antiferromagnetic, and ferromagnetic phases of a thin film of the topological material MnBi2Te4. In each magnetic phase, we extract a charge carrier phase coherence length of about 100 nm. The conductance magnetofingerprint is repeatable when sweeping applied magnetic field within one magnetic phase. Surprisingly, in the antiferromagnetic and canted antiferromagnetic phases, but not in the ferromagnetic phase, the magnetofingerprint depends on the direction of the field sweep. To explain our observations, we suggest that conductance fluctuation measurements are sensitive to the motion and nucleation of magnetic domain walls in MnBi2Te4.

A comparative study of magnetic (Mn) and non-magnetic (In) elements-doped Bi2Se3 topological insulator: Structural, magnetic, transport, and weak anti-localization property exploration

The present comparative study explores the effect of magnetic (Mn) and non-magnetic (In) doping on structural, magnetic, and quantum-transport properties of Bi2Se3 topological insulator. Un-doped as well as Mn, In-doped Bi2Se3 samples have been synthesized using the solid-state reaction method, confirmed by HR-TEM results. The XRD pattern along with Rietveld refinement reveals that, both Mn and In-dopants substitute the Bi-ion together with slightly interstitially incorporation in host Bi2Se3. It is also found that In-doping is more preferable for Bi-substitution than Mn, which is well supported through our XPS analysis. The study of magnetism indicates the diamagnetic nature of un-doped Bi2Se3 gradually transformed into ferromagnetism for high Mn-doping, whereas In-doped Bi2Se3 shows a mixing of diamagnetism and paramagnetism. Electrical-transport establishes that In-doped Bi2Se3 preserves the hostʼs metallic nature; however, the arrest of metallic response is observed through the evidence of Kondo effect below 185 K for high Mn-doping. Interestingly, the Mn-doping creates a transformation of hostʼs weak anti-localization (WAL) towards weak localization (WL), and produces a significant reduction in the phase coherence length (lφ) of host Bi2Se3, verified by the magneto-conductance analysis. Whereas, In-doping shows relatively minor deviation in hostʼs WAL effect for higher doping limit.

Low-temperature resistivity upturn and weak antilocalization in layered Ta1.04Ru0.78Te4 bulk single crystal

Quantum corrections to conductivity, which reflect charge carriers' quantum behavior, are a significant topic in condensed state physics and device design. A resistivity upturn at low temperature or weak antilocalization due to quantum corrections has been often observed experimentally. However, the coexistence of the low-temperature resistivity upturn and weak antilocalization from quantum corrections in bulk single crystals is seldom reported. Here, we report the transport properties of bulk Ta1.04Ru0.78Te4 single crystals. The samples showed a metallic behavior with a resistivity upturn below ∼8.6 K, which may be the result of quantum correction to the resistivity. The magnetic field enhances the upturn feature. The weakly nonlinear Hall resistivity with a positive slope suggests a p-type and multiband feature for bulk Ta1.04Ru0.78Te4; the electron and hole concentrations and mobilities of the samples are very close to each other and have the same order of magnitude. The Ta1.04Ru0.78Te4 single crystals displayed small and positive magnetoresistance, and the 3 K magnetoresistance at 9 T was about 15%. A lack of overlap of Kohler's plot curves at different temperature implies the violation of Kohler's rule. At low temperature, the dip-like magnetoresistance at low field strengths suggests a weak antilocalization in the Ta1.04Ru0.78Te4 single crystal. A small phase coherence length implies weakened screening and enhancing electron–electron interaction effects. These results reveal the quantum transport properties of Ta1.04Ru0.78Te4 single crystals, which can be considered in the future device design.

Spatial coherence length and wave structure function for plane waves transmitted in anisotropic turbulent oceans.

The spatial coherence length and wave phase structure function are two important factors in describing turbulence's effect on light propagation in seawater. This paper derives the wave phase structure function and spatial coherence length of plane waves in moderate to strong turbulent channels by deriving a "modification seawater turbulence power spectrum" and an oceanic-modified Rytov approximation. The evolutions in wave structure function, coherence length with the temperature dissipation rate, energy dissipation rate, anisotropy turbulence factor, signal wavelength, and propagation distance are analyzed by numerical calculation. In the moderate and strong turbulence regions, the phase structure function and spatial coherence length increase and decrease with increasing transmission distance and turbulence strength, respectively, and there is a saturation tendency for both. The fluctuation of seawater salinity has a greater effect on the phase structure function and coherence length than the temperature fluctuation. In addition, the wave structure function decreases with increasing signal wavelength and degree of turbulent anisotropy, but the trend of spatial coherence length is reversed.

Synthesis and characterization of half-Heusler ScPtBi films via three-source magnetron co-sputtering on Nb superconductor buffer layer

The half-Heusler compounds exhibit simultaneous electrical, optical, and magnetic properties hence extensive research efforts have recently been dedicated to their development in diverse applications. Herein, we detail the fabrication and magnetic characterization of thin film ScPtBi was grown at 300 °C on Nb-buffered Si (100) single crystals (Nb/Si (100)) by using magnetron co-sputtering. X-ray diffraction examinations have unveiled the polycrystalline growth nature of the films. It is seen that the obtained ScPtBi thin film is strained by 1.4% compared to the single crystal of ScPtBi. The morphological observations, conducted by scanning electron microscopy (SEM), revealed that the Nb buffer layer has a smooth and crack-free surface. Moreover, it has been observed that the Bi islands are formed on the surface of ScPtBi film, which is confirmed by the high-resolution scanning transmission electron microscopy (HR-STEM) analyses. The presence of the weak antilocalization (WAL) effect has been observed in the samples. The Hikami-Larkin-Nagaoka (HLN) equation was used to analyze experimental data, aiming to calculate the phase coherence length (lφ) and the coefficient α. As the temperature increased, the phase coherence length decreased from 39 nm to 24 nm. The coefficient α was determined to be in the range of 500 to 1100. As a result, it has been concluded that the presence of the WAL effect might be connected to the strong spin-orbit coupling (SOC) nature of the films instead of from topologically protected surface states.

Increased phase coherence length in a porous topological insulator

The surface area of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ thin films was increased by introducing nanoscale porosity. Temperature dependent resistivity and magnetotransport measurements were conducted both on as-grown and porous samples (23 and 70 nm). The longitudinal resistivity of the porous samples became more metallic, indicating the increased surface area resulted in transport that was more surfacelike. Weak antilocalization was present in all samples, and remarkably the phase coherence length doubled in the porous samples. This increase is likely due to the large Fermi velocity of the Dirac surface states. Our results show that the introduction of nanoporosity does not destroy the topological surface states but rather enhances them, making these nanostructured materials promising for low energy electronics, spintronics and thermoelectrics.

Bulk-surface coupling in dual topological insulator Bi1Te1 and Sb-doped Bi1Te1 single crystals via electron-phonon interaction

Recently, has been proved to be a dual topological insulator (TI), a new subclass of symmetry-protected topological phases, and predicted to be higher order topological insulator (HOTI). Being a dual TI (DTI), Bi1Te1 is said to host quasi-1D surface states (SSs) due to weak TI phase and topological crystalline insulating SSs at the same time. On the other hand, HOTI supports topologically protected hinge states. So, is a unique platform to study the electrical signature of topological SS (TSS) of fundamentally different origins. Though there is a report of magneto-transport measurements on large-scale Bi1Te1 thin films, the Bi1Te1 single crystal is not studied experimentally to date. Even the doping effect in a DTI Bi1Te1 is missing in the literature. In this regard, we performed the perpendicular and parallel field magneto-transport measurement on the exfoliated microflake of Bi1Te1 and Sb-doped Bi1Te1 single crystals, grown by the modified Bridgmann method. Our metallic sample shows the weak anti-localization behavior analyzed by the multi-channel Hikami-Larkin-Nagaoka equation. We observed the presence of a pair of decoupled TSS. Further, we extracted the dephasing index (β) from temperature (T)-dependence of phase coherence length (L), following the power law equation (L). The thickness-dependent value of β indicates the transition in the dephasing mechanism from electron-electron to electron-phonon interaction with the increase in thickness, indicating the enhancement in the strength of bulk-surface coupling. Sb-doped system shows weakened bulk-surface coupling, hinted by the reduced dephasing indices.

Emergent quantum transport due to quenched magnetic impurity scattering by antiferromagnetic proximity in SrCuO2/SrIrO3

Through an antiferromagnetic proximity effect, we demonstrate the evidence for quenched magnetic impurity scattering in a spin-orbit-coupled semimetal ${\mathrm{SrIrO}}_{3}$ proximitized with an antiferromagnetic ${\mathrm{SrCuO}}_{2}$ layer from quantum interference originated magnetoconductance study. Two distinct observations, i.e., (i) enhanced effective phase coherence length (${l}_{\ensuremath{\phi}}$) and (ii) emergence of chiral-anomaly-induced topological response in longitudinal magnetoconductance ($\stackrel{P\vec}{B}||\stackrel{P\vec}{E}$), signify that the magnetic impurity scattering is suppressed in the ${\mathrm{SrCuO}}_{2}/{\mathrm{SrIrO}}_{3}$ bilayer. The quenching of magnetic impurity scattering is discussed in the framework of the antiferromagnetic proximity effect, which is originated from spin Andreev reflection at the ${\mathrm{SrCuO}}_{2}/{\mathrm{SrIrO}}_{3}$ interface. This work unfolds a practical means to circumvent the detrimental effect of unintended magnetic impurity scattering and preserve quantum phenomena in complex materials.

Dephasing effect promotes the appearance of quantized Hall plateaus

The quantum Hall effect (QHE) is a topologically protected phenomenon which has been observed in various systems. In experiments, the size of Hall bar device to realize the QHE is generally much larger than the phase coherence length, in which the quantum coherence of electrons is destroyed. Here, we theoretically study the influence of dephasing effect on the quantized Hall (QH) plateaus. We find that the QH plateaus disappear in perfectly quantum coherent systems if the coupling between leads and central region is imperfect. The Hall resistance is very large and strongly oscillates instead of presenting the QH plateaus in this case. However, by introducing the dephasing, Hall resistance decreases and the QH plateaus appear gradually. Similar results can also be observed for the quantum anomalous Hall effect. Our results propose that dephasing effect promotes the appearance of QH plateaus, which opens a new topic of the dephasing effect on topological systems.

Weak localization effect in Zn1−xCdxO/CdO heterostructures

We present a systematic investigation of the morphological and magnetotransport properties of Zn1−xCdxO/CdO heterostructures managing the electronic barrier by changing x values between 0.50 and 0.95. From physical parameters such as roughness and crystallite size obtained through scanning electron microscopy and x-ray diffraction, we established a correlation between the disorder degree and the amplitude of the negative magnetoresistance as well as the nature of the dominating inelastic scattering mechanisms. The magnetoresistance measurements revealed a negative amplitude for all heterostructures, which clearly shows that the weak localization effect is observed in the low temperature range. Applying the three-dimensional weak localization theory (Kawabata 3D), we extract the phase coherence length, and a relatively large value (maximum reaches 135 nm at 4.2 K) is obtained for the Zn0.05Cd0.95O/CdO heterostructure.

Interference Transport in a Two-Dimensional Topological Insulator in a CdHgTe Quantum Well

Interference transport in mesoscopic samples of a two-dimensional topological insulator in CdHgTe quantum wells is studied for the first time. It is established that quasi-ballistic edge transport in such an insulator exists at lengths up to 10 µm. In this transport regime, almost periodic Aharonov–Bohm oscillations caused by the formation of closed loops with a characteristic size of about 200 nm by edge states are found. The phase coherence length in the two-dimensional topological insulator is determined for the first time from the measured temperature dependence of their amplitude.

Selective Area Growth of PbTe Nanowire Networks on InP

AbstractHybrid semiconductor–superconductor nanowires are promising candidates as quantum information processing devices. The need for scalability and complex designs calls for the development of selective area growth techniques. Here, the growth of large scale lead telluride (PbTe) networks is introduced by molecular beam epitaxy. The group IV‐VI lead‐salt semiconductor is an attractive material choice due to its large dielectric constant, strong spin‐orbit coupling, and high carrier mobility. A crystal re‐orientation process during the initial growth stages leads to single crystalline nanowire networks despite a large lattice mismatch, different crystal structure, and diverging thermal expansion coefficient to the indium phosphide (InP) substrate. The high quality of the resulting material is confirmed by Hall bar measurements, indicating mobilities up to 5600 cm2 (Vs)−1, and Aharonov–Bohm experiments, indicating a low‐temperature phase coherence length exceeding 21 µm. Together, these properties show the high potential of the system as a basis for topological networks.

Variable range hopping conductivity in molecular beam epitaxial InSb

A variable range hopping (VRH) transport mechanism can be induced in molecular beam epitaxial, n-type doped InSb wafers with focussed Ga+ ion beam damage. This technique allows areas of wafer to be selectively damaged and then subsequently processed into gated metal–insulator–semiconductor (MIS) devices where a disordered, two-dimensional (2D) device can be established. At high levels of damage (dose >1016 Ga+ ions cm−2) amorphous crystalline behavior results with activated conductivity characteristic of a three-dimensional system with VRH below 150 K. At lower doses (1014–1016 Ga+ ions cm−2) a thermally activated conductivity is induced at ∼0.9 K, characteristic of Mott phonon-assisted VRH. At 1 K the devices either conduct with conductivity >∼(e 2/h) where e is the fundamental charge and h is Planck’s constant, or are thermally activated depending on the dose level. The lightly damaged devices show weak antilocalization signals with conductivity characteristic of a 2D electronic system. As the Ga+ dose increases, the measured phase coherence length reduces from ∼500 nm to ∼100 nm. This provides a region of VRH transport where phase-coherent transport processes can be studied in the hopping regime with the dimensionality controlled by a gate voltage in an MIS-device.

Dynamics of thermalization of two tunnel-coupled one-dimensional quasicondensates

We study the non-equilibrium dynamics of two tunnel-coupled one-dimensional quasicondensates following a quench of the coupling strength from zero to a fixed finite value. More specifically, starting from two independent quasicondensates in thermal equilibrium, with initial temperature and chemical potential imbalance, we suddenly switch on the tunnel-coupling and analyze the post-quench equilibration in terms of particle number and energy imbalances. We find that, in certain parameter regimes, the net energy can flow from the colder quasicondensate to the hotter one and is governed by the surplus of low energy particles flowing from the cold to the hot system relative to the high-energy particles flowing in the reverse direction. In all cases, the approach to the new thermal equilibrium occurs through transient, damped oscillations. We also find that for a balanced initial state the coupled quasicondensates can relax into a final thermal equilibrium state in which they display a thermal phase coherence length that is larger than their initial phase coherence length, even though the new equilibrium temperature is higher. The increase in the phase coherence length occurs due to phase locking which manifests itself via an increased degree of correlation between the local relative phases of the quasicondensates at two arbitrary points.

Tuning weak localization in single-layer disordered SnSe2/graphene/h-BN field-effect device

We report fabrication and measurements of single-layer SnSe2/chemical vapor deposition (CVD) graphene/hexagonal boron nitride (h-BN) field-effect device. The coherent magnetotransport properties of such a hybrid system are systematically studied so as to obtain a good understanding of the structure which may find potential applications in thermoelectricity, flexible electronics, quantum coherent sensor as well as stress sensing. We observed weak localization well described by the Hikami-Larkin-Nagaoka model and the phase coherence length is around 540 nm for V BG = −20 V at 1 K. The phase coherence length could be effectively changed by controlling the temperature and gate voltage. We also obtain good field-effect dependent properties of atomic-scale SnSe2 ultrathin film/graphene system. Given the current challenges in tuning single-layer SnSe2/CVD graphene on h-BN with a suitable dielectric layer, our results suggest the potential of quantum coherent effect, an effective way for development of future quantum nano-switch device.

Evidence of decoupling of surface and bulk states in Dirac semimetal Cd3As2

Dirac semimetals have attracted a great deal of current interests due to their potential applications in topological quantum computing, low-energy electronic devices, and single photon detection in the microwave frequency range. Herein are results from analyzing the low magnetic (B) field weak-antilocalization behaviors in a Dirac semimetal Cd3As2 thin flake device. At high temperatures, the phase coherence length l ϕ first increases with decreasing temperature (T) and follows a power law dependence of l ϕ ∝ T −0.4. Below ∼3 K, l ϕ tends to saturate to a value of ∼180 nm. Another fitting parameter α, which is associated with independent transport channels, displays a logarithmic temperature dependence for T > 3 K, but also tends to saturate below ∼3 K. The saturation value, ∼1.45, is very close to 1.5, indicating three independent electron transport channels, which we interpret as due to decoupling of both the top and bottom surfaces as well as the bulk. This result, to our knowledge, provides first evidence that the surfaces and bulk states can become decoupled in electronic transport in Dirac semimetal Cd3As2.

Observation of Aharonov-Bohm effect in PbTe nanowire networks

We report phase coherent electron transport in PbTe nanowire networks with a loop geometry. Magnetoconductance shows Aharonov-Bohm (AB) oscillations with periods of $h/e$ and $h/2e$ in flux. The amplitudes of $h/2e$ oscillations are enhanced near zero magnetic field, possibly due to interference between time-reversal paths. Temperature dependence of the AB amplitudes suggests a phase coherence length $\ensuremath{\sim}8\text{}12\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{m}$ at 50 mK. This length scale is larger than the typical geometry of PbTe-based hybrid semiconductor-superconductor nanowire devices.