Electron Dephasing Research Articles

Probing surface states: A study of UCF and WAL in Bi1.9Sb0.1Te2Se topological insulator

In the exploration of three-dimensional quaternary topological insulators, understanding surface states has become pivotal for unraveling the underlying physics and tapping into potential applications. Our study delves into the temperature and magnetic field-angle dependence of universal conductance fluctuations (UCF) and weak anti-localization (WAL) effects in a Bi1.9Sb0.1Te2Se topological insulator-based mesoscopic device. Conventionally, other low-temperature transport phenomena in probing surface states may inevitably face interference from three-dimensional bulk conductance. However, we experimentally demonstrate that, at low temperatures, UCF reflects the properties of two-dimensional topological surface states more accurately, thereby providing a more reliable and distinct way to confirm their existence. Moreover, we carefully analyze the temperature-dependent WAL using the Hikami–Larkin–Nagaoka model, proposing a crucial role for charge puddles associated with electrostatic fluctuations in the electron dephasing process. Our findings not only emphasize the key role of UCF in unveiling the underlying behavior of topological surface states but also offer a deeper understanding of phase-coherent transport in quaternary topological insulators.

Spin Coherence and Spin Relaxation in Hybrid Organic-Inorganic Lead and Mixed Lead-Tin Perovskites.

Metal halide perovskites make up a promising class of materials for semiconductor spintronics. Here we report a systematic investigation of coherent spin precession, spin dephasing and spin relaxation of electrons and holes in two hybrid organic-inorganic perovskites MA0.3FA0.7PbI3 and MA0.3FA0.7Pb0.5Sn0.5I3 using time-resolved Faraday rotation spectroscopy. With applied in-plane magnetic fields, we observe robust Larmor spin precession of electrons and holes that persists for hundreds of picoseconds. The spin dephasing and relaxation processes are likely to be sensitive to the defect levels. Temperature-dependent measurements give further insights into the spin relaxation channels. The extracted electron Landé g-factors (3.75 and 4.36) are the biggest among the reported values in inorganic or hybrid perovskites. Both the electron and hole g-factors shift dramatically with temperature, which we propose to originate from thermal lattice vibration effects on the band structure. These results lay the foundation for further design and use of lead- and tin-based perovskites for spintronic applications.

Trajectory-controlled high-order harmonic generation in ZnO crystals.

We experimentally and theoretically study high-order harmonic generation in zinc oxide crystals irradiated by mid-infrared lasers. The trajectories are mapped to the far field spatial distribution of harmonics. The divergence angles of on-axis and off-axis parts exhibit different dependences on the order of the harmonics. This observation can be theoretically reproduced by the coherent interference between the short and long trajectories with dephasing time longer than 0.5 optical cycle. Further, the relative contribution of the short and long trajectories is demonstrated to be accurately controlled by a one-color or two-color laser on the attosecond time scale. This work provides a reliable method to determine the electron dephasing time and demonstrates a versatile control of trajectory interference in the solid high-order harmonic generation.

Plasma density transition-based electron injection in laser wake field acceleration driven by a flying focus laser

By using a high-intensity flying focus laser, the dephasingless [Phys. Rev. Lett. 124 134802 (2020)] or phase-locked [Nat. Photon. 14 475 (2020)] laser wakefield acceleration (LWFA) can be realized, which may overcome issues of laser diffraction, pump depletion, and electron dephasing which are always suffered in usual LWFA. The scheme thus has the potentiality to accelerate electrons to TeV energy in a single acceleration stage. However, the controlled electron injection has not been self-consistently included in such schemes. Only external injection was suggested in previous theoretical studies, which requires other accelerators and is relatively difficulty to operate. Here, we numerically study the actively controlled density transition injection in phase-locked LWFA to get appropriate density profiles for amount of electron injection. The study shows that compared with LWFA driven by lasers with fixed focus, a larger plasma density gradient is necessary. Electrons experience both transverse and longitudinal loss during acceleration due to the superluminal group velocity of the driver and the variation of the wakefield structure. Furthermore, the periodic deformation and fracture of the flying focus laser in the high-density plasma plateau make the final injected charge also depend on the beginning position of the density downramp. Our studies show a possible way for amount of electron injection in LWFA driven by flying focus lasers.

Electron dephasing processes in the thin and thick Ta-doped SnO2 films

We have systematically investigated the electron dephasing processes of the thin and thick Ta-doped SnO2 films. All samples exhibit electron transport characteristics of degenerate semiconductors. At low temperature T (below ∼70 K), the weak-localization-induced negative magnetoresistance (MR) is observed. For the thick films with the thicknesses of ∼300 nm, the observed MR can be well described by three-dimensional weak-localization theory. The electron dephasing rate 1/τφ varies linearly with T3/2, indicating that the electron–electron scattering dominates the electron dephasing process. This is different from the common observation that the electron–phonon scattering is the main dephasing process for electrons in three-dimensional disordered conductors. For the thin films with the thicknesses of ∼30 nm, the MR can be well described by two-dimensional weak-localization theory. By analyzing the electron dephasing rate as a function of T, we find that both large- and small-energy-transfer electron–electron scattering processes play an important role in the electron dephasing.

A home-made portable device based on Arduino Uno for pulsed magnetic resonance of NV centers in diamond

We describe the realization of a homemade and portable setup to perform experiments of pulsed magnetic resonance of nitrogen-vacancy (NV) centers in diamonds. The system is fully implemented by using an Arduino Uno board equipped with an AVR microcontroller that is used as a transistor-transistor logic pulse sequencer to drive precise laser and microwave pulses with a resolution of 62.5 ns. The equipment is assembled with low-cost modules on a printed circuit board and placed in a compact box with a volume of 20 × 40 × 10 cm3. The detection system is based on a switched integrator and a photodiode in the vicinity of a diamond substrate and read by oversampling the analog-to-digital converter of Arduino Uno. We characterize a CVD diamond sample by performing the pulsed optically detected magnetic resonance and we show the possibility to perform a coherent manipulation of the electron spin of NV centers by driving Rabi oscillations up to 6 MHz with microwave powers within 1 W. We demonstrate different pulse sequences to study electron spin relaxation and dephasing. Finally, we propose additional modules and an antenna to perform the multifrequency manipulation of the electron spin by microwave and radio-frequency pulses. Compared to the previous studies, our system results in a low-cost setup with significantly reduced complexity, which finds application as a learning module for science education and enables a wider audience to access the magnetic resonance in diamond.

Atomistic Band-Structure Computation for Investigating Coulomb Dephasing and Impurity Scattering Rates of Electrons in Graphene.

In this paper, by introducing a generalized quantum-kinetic model which is coupled self-consistently with Maxwell and Boltzmann transport equations, we elucidate the significance of using input from first-principles band-structure computations for an accurate description of ultra-fast dephasing and scattering dynamics of electrons in graphene. In particular, we start with the tight-binding model (TBM) for calculating band structures of solid covalent crystals based on localized Wannier orbital functions, where the employed hopping integrals in TBM have been parameterized for various covalent bonds. After that, the general TBM formalism has been applied to graphene to obtain both band structures and wave functions of electrons beyond the regime of effective low-energy theory. As a specific example, these calculated eigenvalues and eigen vectors have been further utilized to compute the Bloch-function form factors and intrinsic Coulomb diagonal-dephasing rates for induced optical coherence of electron-hole pairs in spectral and polarization functions, as well as the energy-relaxation time from extrinsic impurity scattering of electrons for non-equilibrium occupation in band transport.

Weak Localization in Polycrystalline Tin Dioxide Films.

The electrical and magnetotransport properties of nanocrystalline tin dioxide films were studied in the temperature range of 4–300 K and in magnetic fields up to 8 T. SnO2−δ films were fabricated by reactive direct current (DC) magnetron sputtering of a tin target with following 2 stage temperature annealing of synthesized samples. The nanocrystalline rutile structure of films was confirmed by X-ray diffraction analysis. The temperature dependences of the resistance R(T) and the negative magnetoresistance (MR) were explained within the frame of a model, taking into account quantum corrections to the classical Drude conductivity. Extracted from the R(T) and R(B) dependences electron dephasing length values indicate the 3D character of the weak localization (WL) in our samples.

Plasma Wave Acceleration of Electron in Bubble Regime in Presence of a Planar Wiggler

The plasma wave acceleration of electron in the bubble regime is investigated in a new configuration containing a planar wiggler. The space-charge field of the laser-created ion channel can focuse and stabilize the electron trajectory to guide it toward the acceleration region. The high-gradient plasma wave field can resonantly accelerate the trapped electron to higher energies in the presence of a planar wiggler compensating the electron dephasing. The results show that in the lower plasma wave amplitudes the planar wiggler plays a more significant role on the electron energy enhancement. The increment of the electron energy in this configuration isvalidated using a three-dimension single-particle code. The energy gain of electrondependency on the planar wiggler, ion channel field, plasma wave angle and amplitude as well as the initial energy of electron has been investigated. The results of paper will be of importance in the optimization of electron energy and improving the quality of the accelerated electrons in the plasma wakefield accelerators.

Unveiling Electron Optics in Two-Dimensional Materials by Nonlocal Resistance Mapping.

We propose a technique based on nonlocal resistance measurements for mapping transport in electron optics experiments. Utilizing tight-binding transport methods, we show how to use a four-terminal measurement to isolate the ballistic transport from a single lead of interest and reconstruct its contribution to the local density of states. This enables us to propose an experimentally tractable four-terminal device with via contacts for measuring Veselago lensing in a graphene p-n junction. Furthermore, we demonstrate how to extend this method as a scanning probe technique, implementing mapping of complex electron optics experiments including angled junctions, collimation optics, and beam steering. Our results highlight the fundamental importance of electron dephasing in ballistic transport and provide guidelines for isolating electron optics signals of interest. These findings unveil a fresh approach to performing electron optics experiments, with a plethora of two-dimensional material platforms to explore.

Quantum Chip with the Optimized Tunnel Structure for Measuring a Charge Qubit Based on a Double Quantum Dot

A circuit of a measuring chip for determining an arbitrary pure state of a charge qubit is proposed. The strength of the current flowing through a single-electron transistor in the steady-state mode depends on the state of the qubit. To enhance the sensitivity of the transistor, its working part is built from three quantum dots (QDs) with the energy levels forming a symmetric configuration. The parameters of the system are calculated using a microscopic model of two-dimensional QDs. The time dependences of the population of the structure’s states are obtained and the current strength, sensitivity, and measuring contrast as functions of the geometric parameters of the system are determined. The effect of dissipative processes related to the acoustic phonons on the measurements is investigated and the rates of electron relaxation and dephasing in a two-level system are calculated.

Spin dephasing of electrons and holes in isotopically purified ZnSe/(Zn,Mg)Se quantum wells

The coherent spin dynamics of resident electrons and holes in an isotopically purified ZnSe/(Zn,Mg)Se single quantum well is investigated in different regimes, requiring corresponding adaption of the applied time-resolved pump-probe Kerr rotation technique. The purification of the Zn and Se atom species in the crystal to the isotopes with zero nuclear spin is expected to lead to an extension of the spin dephasing times of resident carriers, due to the suppression of their interaction with the nuclear spins. Indeed, we find no indication of carrier-nuclear interaction in this sample and link the observed carrier spin relaxation to the spin-orbit interaction. Theoretical considerations support the experimental results.

Comparative Analysis of Phenomenological and Numerical Modeling of Experimentswith Single-Pass Free-Electron Lasers

We model the evolution of radiation harmonics in a single-pass free-electron laser using a phenomenological model, in which the main losses are taken into account separately for each of the harmonics. The modeling results are compared with the corresponding FEL experiments and the simulations, which we performed using the PERSEO code. A wide wavelength range (0.15–500 nm) is considered. The phenomenological description based on the use of several basic FEL parameters, such as the electron beam current, average energy, energy spread, and emittance of electrons, yields the results that correspond well to the LEUTL, SPARC, and LCLS experiments in various conditions. The evolution of the radiation power, bunching, and electron energy spread in the Spring 8 FEL at the follow-up stage is also considered. The necessity to improve electron beam parameters for generation of harmonics in this FEL is shown. The phenomenological model allows assessing the operation of the available and future FELs fast. Using it, one can model FELs with nearly any undulator with allowance for higher harmonics of their magnetic fields. Moreover, this model makes it possible to describe easily various effects in designed FELs, e.g., those with dephasing of electrons and photons, filtering of harmonics, and other features.

Observation of multichannel quantum coherent transport and electron-electron interaction in Bi2Te3 single crystal

The bulk of topological insulators is relatively unexplored, due to the excess contribution of conduction from native defects. Here, we investigate the bulk conduction in a Bi2Te3 crystal having a reduced number of defect states. Our results reveal the presence of three transport regimes which are dominated by thermal activation across the bulk band gap, defect state charge conduction, and quantum coherent transport. The low temperature conductance and magnetoconductance reveal the presence of multichannel two-dimensional quantum coherent transport in the bulk. The number of channels is of the order of quintuple layers, signifying each quintuple layer as a single transport channel. These transport channels exhibit a two-dimensional electron-electron interaction effect causing electron dephasing, whereas the conduction from defect states exhibits a three-dimensional electron-electron interaction effect.

Nonlinear absorption of ultrashort ultrahigh intensity laser pulses in fullerene

Non linear absorption of intense few cycle laser pulses in fullerenes is studied by treating it as a laser driven nonlinear oscillator, using a rigid shell model. Efficient collisionless absorption of intense (Intensity I < 1018W/cm2) ultrashort laser pulses in fullerene (C60) is estimated owing to its extraordinary anharmonicty which is quite different than the conventionally used solid gas clusters. Compared to gas clusters nonlinear resonance in C60 has unusual characteristics. Factors other than nonlinear resonance which contribute in energy absorption in C60 are outlined. Strong dephasing of electrons with the laser field both at onset and during the dynamical oscillations leads to the efficient energy gain by the electrons. It is found that fullerene has a distinct threshold intensity requirement depending on its charge for efficient absorption. These properties make fullerene unique for efficient absorption of high intensity few cycle laser pulses.

Electrical transport properties of polycrystalline SnO2 thin films

We study the electrical transport properties of degenerate tin dioxide thin films with thickness larger than 250 nm. Our samples have a rutile structure and the effective mass of electron is 0.31 m0, which is obtained from the variation in optical bandgap. The temperature dependence of the Hall mobility indicates that the ionized impurity scattering is the dominant elastic scattering mechanism for electrons. In the low temperature range, the clear negative magnetoresistivity is observed, which can be attributed to the three-dimensional weak localization (WL) effect. By applying the three-dimensional WL theory, we extracted the electron dephasing length, which decreased on increasing temperature. Unexpectedly, the temperature dependence of the extracted dephasing length is found to be dominated by the electron-electron scattering in the small-energy-transfer process and the electron-phonon scattering has negligible contribution. This can be attributed to the low electron concentration in our samples.

Elements of a dielectric laser accelerator

We experimentally demonstrate several physical concepts necessary for the future development of dielectric laser accelerators—photonic elements that utilize the inelastic interaction between electrons and the optical near fields of laser-illuminated periodic nanostructures. To build a fully photonic accelerator, concatenation of elements, large energy gains, and beam steering elements are required. Staged acceleration is shown using two spatio-temporally separated interaction regions. Further, a chirped silicon grating is used to overcome the velocity dephasing of subrelativistic electrons with respect to its optical near fields, and last, a parabolic grating geometry serves for focusing of the electron beam.

Disorder dependence electron phonon scattering rate of V82Pd18 − xFex alloys at low temperature

We have systematically investigated the disorder dependence electron phonon scattering rate in three dimensional disordered V82Pd18−xFex alloys. A minimum in temperature dependence resistivity curve has been observed at low temperature T=Tm. In the temperature range 5 K≤T≤Tm the resistivity correction follows ρo5/2T1/2 law. The dephasing scattering time has been calculated from analysis of magnetoresistivity by weak localization theory. The electron dephasing time is dominated by electron–phonon scattering and follows anomalous temperature (T) and disorder (ρ0) dependence behaviour like τe-ph−1∝T2/ρ0, where ρ0 is the impurity resistivity. The magnitude of the saturated dephasing scattering time (τ0) at zero temperature decreases with increasing disorder of the samples. Such anomalous behaviour of dephasing scattering rate is still unresolved.

Multi-color, femtosecond [formula omitted]-ray pulse trains driven by comb-like electron beams

Photon engineering can be exploited to control the nonlinear evolution of the drive pulse in a laser–plasma accelerator (LPA), offering new avenues to tailor electron beam phase space on a femtosecond time scale. One promising option is to drive an LPA with an incoherent stack of two sub-Joule, multi-TW pulses of different colors. Slow self-compression of the bi-color optical driver delays electron dephasing, boosting electron beam energy without accumulation of a massive low-energy tail. The modest energy of the stack affords kHz-scale repetition rate at manageable laser average power. Propagating the stack in a pre-formed plasma channel induces periodic self-focusing in the trailing pulse, causing oscillations in the size of accelerating bucket. The resulting periodic injection generates, over a mm-scale distance, a train of GeV-scale electron bunches with 5D brightness exceeding 1017A∕m2. This unconventional comb-like beam, with femtosecond synchronization and controllable energy spacing of components, emits, via Thomson scattering, a train of highly collimated gigawatt γ-ray pulses. Each pulse, corresponding to a distinct energy band between 2.5 and 25 MeV, contains over 106 photons.

Enhanced electron dephasing in three-dimensional topological insulators

Study of the dephasing in electronic systems is not only important for probing the nature of their ground states, but also crucial to harnessing the quantum coherence for information processing. In contrast to well-studied conventional metals and semiconductors, it remains unclear which mechanism is mainly responsible for electron dephasing in three-dimensional topological insulators (TIs). Here, we report on using weak antilocalization effect to measure the dephasing rates in highly tunable (Bi,Sb)2Te3 thin films. As the transport is varied from a bulk-conducting regime to surface-dominant transport, the dephasing rate is observed to evolve from a linear temperature dependence to a sublinear power-law dependence. Although the former is consistent with the Nyquist electron-electron interactions commonly seen in ordinary 2D systems, the latter leads to enhanced electron dephasing at low temperatures and is attributed to the coupling between the surface states and the localized charge puddles in the bulk of 3D TIs.